|Had to edit this post because the picture I found disappeared, but I found a better one anyway.
Here's another generic pic I found showing a Smiths tach after removal, so you get a better idea of the way the retaining fingers work. Ignore the wiring etc as it's from a completely different model, but the retainers work the same way. You can see how the knurled nut tightens the retaining fingers, basically squeezing the dash cutout between the tach bezel and the retaining fingers.
Last edited on 12-01-2014 11:48 pm by answerman